Talking from her home in New York, she explains how even the moistness of our earwax is encoded in our genes, why our decisions are not entirely our own, and how the genetic imprint of distant historical events like slavery can shape attitudes today.

The book begins with you flying to Australia, about to discover a long-kept family secret. Tell us about the Kenneallys.

I kept circling around this memory from when I was in grade two, about eight years old. Our teacher explained what a family tree was. I was completely bewitched by this idea. I thought it was magical that you could see these lines through your family—who came from whom.

So I ran home excitedly to ask my parents what the names of my grandparents were. I was really surprised by their response. They were not happy about it. In fact, they were kind of indignant. Their attitude was: What business is it of your teacher to ask these sort of things?

Many years later, when I was in my early 20s, we were having a conversation around the kitchen table, and my father told us that the person we thought of as our grandfather was really his grandfather. And that the woman he'd grown up thinking was his sister was really his mother. And that he didn't know who his father was.

It was an incredibly significant moment in our lives. Not for the reasons I think my father feared. He came from a generation when illegitimacy was a terribly shameful thing. But this was the late 20th century. We didn't have any of those feelings. But we did feel this shuddering in our identity. The thing we'd always thought was true was not true. That's what connected with me with these questions I had about how our identities are built—what gets passed down to us over the years. And what can you ever really know?

You write that "the question that came to concern me, and that lies at the heart of this book, is how many decisions and how much of our self-knowledge are ultimately path dependent." Unpack that idea for us.

We have this vision of ourselves as completely in control of who we are in any one moment, as essentially creating ourselves—at least once we've become adults.

But there are many traits we have and many decisions we come to that are shaped by paths we've taken in our lives and by paths our ancestors have taken.

Evolution really is a past-dependent process. What exists now has evolved from what came before. This principle also applies to our personal history, our family history, and biology, because our genes are passed down to us.

"Genealogy is bunk," you quote one science writer as saying. But that doesn't seem to stop more and more people searching out their ancestors.

That's absolutely right. And that's because people who are searching out their ancestors are onto something. That was one of the most fascinating parts of writing this book. When I first started talking about it to people, I kept coming up against this attitude, which was very perplexing to me, because genealogy is one of the most popular hobbies in the world.

At the same time there's this widely held notion that genealogy is a ridiculous, self-indulgent pastime. I think that comes from a few different things. First I think it comes from the misuse of genealogy and our ideas of inheritance. Not just in the eugenics of Nazi Germany. Many other countries had ideas about lineage and genealogy, and biology, which they believed made them superior to others. Some pockets of the world are also still very much class-based, and people don't want to return to that.

There's also a notion in America whereby we want to see ourselves as completely in charge of who we are. We don't want to think of ourselves as having been shaped by the past.

One of the most fascinating things in your book was the idea that the legacy of historical events like slavery or the Black Death can shape our ability to trust people—and the success of a society. Tell us about the work of Nathan Nunn.

Nathan Nunn is a pioneer among a group of economists who are using big data to look at the impact of historical events on attitudes of today. With another economist, Leonard Wantchekon, he conducted this incredible study whereby they looked at levels of interpersonal trust in Africa today.

They asked if slavery had any kind of impact. What they found was that there was a correlation between regions where more slaves were taken and lower levels of trust today.

Wantchekon is a Princeton economist now, but he grew up in Benin, West Africa, one of the main slave exporting centers. And when the results of their research became known in Benin, there was a huge response. Many people got in touch with him and gave him these heartfelt acknowledgments that he'd identified something that was real—and was shaping their lives today.

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We'll know more about inheritance through the ages once we fully understand the genetics of the face, says author. A girl with Korean and Hispanic heritage is pictured above.

Photograph by Martin Schoeller, National Geographic

We think of a country like the U.S. as a melting pot where people from different cultures blend together by adopting the values of their new homeland. But your research shows that immigrants often reproduce old values even their ancestors have left behind.

Absolutely. The myth of American independence as complete abandonment of the Old World is not true. People very much bring the Old World with them. One study looked at how many children women from different cultures were likely to have.

And what they discovered was that they're influenced by the numbers of children their grandparents had, by the choices their grandparents made, even if they've never met those grandparents or been back to the Old World.

Obviously, their grandparents' children, their own parents, bring those values with them, and it influences their life choices. I love that, because we think of these choices as so personal and so completely independent. They're choices you make by yourself or with your partner. But no matter what you tell yourself, what your grandparents were up to also seems to affect you too.

You say that "DNA studies in Britain switched the light on the Dark Ages." How?

That's this fantastic study by geneticists at Oxford University, which looked across the genome of over 2,000 people in England. Normally with studies like that when we're looking for evidence of history in the genome, people look at whether someone has a marker or a set of markers that tells us where their ancestors came from, or where some of their ancestors came from.

What these guys did, though, was look at patterns across the genome. By doing that, they were able to drill down in much finer detail than anyone has ever done before and see what was going on 1,000 or 2,000 years ago.

They discerned differences in small populations, which were as closely related geographically as people who lived in Cornwall and Devon. They took those differences between those fine-grain groups and correlated them with historic events like the Dark Ages.

You say that "the moistness of our earwax can be traced to a single letter within a single gene"!

[Laughs] Much of the shape of our body comes directly from our genome. But we're just starting to work that out. Earwax is a single genetic marker. But a lot of our physical traits will be shaped not just by one marker or in one gene but by many genes working together.

The genetics of the face is fascinating. You know when you see someone who looks like someone else, but you can't quite put your finger on it? Well in the last year or two scientists have identified five different genes that clearly contribute, though not necessarily exclusively, to the way our faces are structured.

And I think we're going to know so much more about inheritance through the ages and be able to understand our own myths about inheritance once we fully understand the genetics of the face.

One of the most touching scenes in the book is when your whole family does a DNA test together. What did you find out about yourselves?

My husband's mother had died young, of a particularly devastating form of multiple sclerosis. And throughout my husband's life, in the back of his mind, had been this question: Would he also develop this disease and perhaps pass it to our children?

So, the first thing he looked at was whether he had the genetics associated with multiple sclerosis. Luckily, as far as current knowledge goes, he does not. It was a huge relief to him. This burden, which he didn't even realize he'd been carrying for all this time, had suddenly lifted.

How close are we to being able to silence specific genes, to prevent hereditary diseases, like Huntington's?

Possibly quite close. There are a lot of teams around the world working very hard to do just that. I think we're a few years away from a drug that can be tested on humans. The initial discovery of the Huntingdon's mutation has been a long time coming. But we wouldn't be anywhere near a cure if the genetic information hadn't been uncovered.

You use the phrase "the politics of DNA"—how information we discover may shatter our illusions about ourselves or our cultural heroes. Give us some examples.

There are stories about people who do their genetic genealogy and expose some secret in their family that can potentially be very distressing. So there's this idea that your genome is a kind of Pandora's box. Be careful looking inside, you never know what might come out.

But as I did my research and spoke to many people who have done this, I felt there were many more stories of positive discovery and revelation, and a willingness to embrace the complications that come up when we look into our past. Genetic genealogy may be the most efficient way that we have now of uncovering secrets, but the distress comes from the secrets, not the method with which they are revealed.

The most famous case in American history is, of course, Thomas Jefferson and Sally Hemings. It's an incredibly important story for American history and for genetics. What DNA tests showed was that Jefferson had fathered many of Sally Hemings's children, something which had been denied for a very long time. And it was denied because people had paid attention to the stories told by white landowners, not the stories told by slaves. The slaves had been telling these stories for a long time. But no one was listening. The genetics proved that it really had happened, and that it wasn't uncommon at the time.

How did writing this book change your life?

That's the hardest question. I would say that it changed my life very much. Personally, it was a really hard book to write, because it involved asking my father questions that I would've been a lot more comfortable not asking.

Every time I thought about what these historic figures had been through, it would push me to go back to my own family. It's a much smaller-scale example of that kind of challenge, but I deeply felt how hard it is to turn around and ask your parents about the past—the past they'd rather not talk about.

It changed this lifelong conversation between my father and myself. There are more truths spoken between us because of this book. And my admiration for him has grown enormously. There were things that he just hasn't wanted to talk about, but however uncomfortable it's been, he's responded and extended himself. I'm still trying to find out information about my father's father. So it's an ongoing process.